This invention relates to a muting apparatus used with an acoustic instrument such as a tape recorder.
With a tape recorder arranged for carrying out microphone recording, a power amplifier is generally subjected to a certain degree of muting during the recording mode. The reason is that if the power amplifier is actuated during the microphone recording, then howling undesirably arises, unless the sound volume is suppressed. Such howling tends to take place in a portable tape recorder in which a microphone and a speaker are mounted in the same cabinet. Further, the recent commercially available tape recorders are often required to have a tape end alarm-sounding function. This is because such alarm can inform the user that the run of a magnetic tape has been brought to an end during recording, even though no watch is kept on the remaining supply of the magnetic tape.
When, with a tape recorder having the aforementioned recording-muting function and tape end alarm-sounding function, a tape end is detected during recording, then it is necessary to release the recording muting and apply muting to the microphone amplifier. Unless a power amplifier is actuated by the release of recording muting, no alarm is sounded from a speaker. If, however, the microphone is actuated during the release of recording muting, then an alarm sound is positively fed back from the speaker to the microphone, giving rise to howling all the same. To prevent howling, therefore, muting is applied to the microphone amplifier almost at the same time as when the recording muting is released.
FIG. 1 shows the arrangement of a tape recorder having the aforesaid recording-muting function and tape end alarm-sounding function. An operation key 10 is connected to a tape recorder control circuit 12. When a recording mode is designated by the operation of the key 10, then the tape recorder control circuit 12 is set for the recording mode. At this time, the tape recorder control circuit 12 issues a signal commanding a constant speed tape run to a tape transport 14. At the issue of said command, the tape recorder control circuit 12 actuates a recording amplifier 16 including a bias circuit (not shown) by means of a recording mode signal E16. At the same time, the tape recorder control circuit 12 supplies a recording-muting signal Er to a power amplifier 18. As a result, the power amplifier 18 is rendered inactive, preventing any sound from being produced from a speaker 20. Input terminals of the recording amplifier 16 and the power amplifier 18 are connected to a signal mixer 22. The signal mixer 22 receives a voice signal E24 which is picked up by a microphone 26 and amplified through a preamplifier (microphone amplifier) 24.
When the run of a recording tape 28 by the tape transport 14 is brought to an end, then the rotation of a takeup reel 30 stops. The takeup reel 30 is coupled to a rotation sensor 32 which issues a pulse signal E32 corresponding to the rotation of the takeup reel 30. The pulse signal E32 is supplied to a tape end detector 34. When the interval of respective pulses constituting the signal E32 is extended over a prescribed value by the stop of the takeup reel 30, then the detector 34 issues the tape end signal E34, which in turn is delivered to the control circuit 12. The control circuit 12 is supplied with an alarm signal E36a issued from an oscillator 36. Upon receipt of the tape end signal E34, the control circuit 12 sends an end alarm signal Ea corresponding to the alarm signal E36a. The end alarm signal Ea represents an audible sound having a frequency of hundreds of Hz or several kHz, and is supplied to the signal mixer 22.
The control circuit 12 generates the end alarm signal Ea and also supplies a tape end muting signal Ee to the preamplifier 24, rendering the preamplifier 24 inactive. That is, when a tape end is detected during the microphone recording, then the power amplifier 18 is released from the recording muting. Instead, muting is applied to the preamplifier 24, and the end alarm signal Ea is supplied to the mixer 22. As a result, the speaker 20 sends a tape end alarm sound corresponding to the end alarm signal Ea. Since, at this time, the preamplifier 24 is rendered inactive, the acoustic feedback loop (positive feedback loop) is not formed which includes the speaker 20 and microphone 26. Therefore, when an end alarm is sounded, no howling is produced.
To attain the above-mentioned muting function, the tape recorder control system 38 and audio amplifiers 18, 24 should be connected via the three kinds of signals Er, Ee, Ea. When, however, it is attempted to produce said three signals Er, Ee, Ea when additionally, the control system is constructed, for example, by a one-chip IC package, then various difficulties sometimes arise. Now let it be assumed that the control system 38 is converted into the IC form, for example, by a dual inline type mold package, and further that when the signals Er, Ee, Ea are not provided, said IC package is received in a 22-pin type package. If, in this case, the IC package is additionally provided with terminals for handling said signals Er, Ee, Ea, then the IC package should be made into a 26-pin type, because external pins are increasingly provided each time by 2 units. When the IC package is changed from the 22 pin type to the 26-pin type, then the problem arises that the IC package is increased in cost, and difficulties are presented in loading of the electric system of a tape recorder reduced in size.